1. Field of the Invention
The present invention relates to a current controlled CR oscillator circuit, and more specifically, to a CR oscillator circuit that achieves small current consumption and a small occupied area.
2. Description of the Related Art
As an oscillator circuit to be used in an electronic device, a crystal oscillator circuit has hitherto been used, in which a semiconductor integrated circuit and a quartz resonator are combined. In the crystal oscillator circuit, when a parasitic capacitance exists near the semiconductor integrated circuit or the quartz resonator, an oscillation frequency may sometimes be deviated from a set value.
Incidentally, in recent years, electronic components have often been mounted at high density owing to a demand for downsizing of the electronic device, and hence the parasitic capacitance existing near the semiconductor integrated circuit or the quartz resonator has tended to be large. In view of this, as an oscillator circuit independent of the mounted state of the integrated circuit, a CR oscillator circuit has hitherto been known.
FIG. 6 is a circuit diagram for illustrating a related-art CR oscillator circuit.
The related-art CR oscillator circuit includes voltage comparators X1 and X2, reference voltage circuits VH and VL, constant current sources I1 and I2, switches S1 and S2, and a capacitor C.
The related-art CR oscillator circuit compares a triangular wave voltage, which is generated by the constant current sources I1 and I2 and the capacitor C, to reference voltages VH and VL by the voltage comparators X1 and X2, to thereby serve as a CR oscillator circuit configured to oscillate a voltage having upper and lower peaks corresponding to the reference voltages VH and VL.
The related-art CR oscillator circuit includes two voltage comparators and thus has a problem of increasing current consumption and an occupied area.
Moreover, an offset voltage is always generated at the voltage comparator, and hence the triangular wave voltage is compared to a voltage different from the set reference voltage, which leads to an oscillation frequency error. Further, the offset voltages of the two voltage comparators do not necessarily have constant voltage values, and hence it is difficult to predict the oscillation frequency error.
In general, a circuit including MOS transistors configured as a differential pair is often used as the voltage comparator. Now, the offset voltage of the voltage comparator is considered. The offset voltage is generated mainly when threshold voltages of the MOS transistors forming the differential pair configuration are deviated from each other. When the threshold voltages of the MOS transistors forming the differential pair configuration are respectively represented by VTH1 and VTH2, a difference ΔVTH between those threshold voltages is generally expressed by Expression 1.ΔVTH=α×tox/√(W×L)  (1)
From Expression 1, it is found that increasing areas of the MOS transistors is a simple method for reducing the difference ΔVTH between the threshold voltages. However, the thickness tox of an oxide film is varied depending on manufacturing conditions, and hence simply increasing the areas of the transistors reduces the difference ΔVTH between the threshold voltages, but cannot keep the value constant. In addition, due to the large area of the transistor, a gate capacitance of the MOS transistor becomes larger as compared to the capacitor for generating a triangular wave voltage. As a result, increasing the areas may be a cause of the oscillation frequency error.